Network analyzer systems enable complete characterization of a network through measurement of the magnitude and phase of its input and transfer characteristics. Typically, these systems include a display device to present a graphical vector representation of the network characteristics. See, for example, U.S. Pat. No. 3,371,273 of Andrew Alford entitled "Automatic Impedance Measuring System Utilizing a Hybrid-Like Network Operating Over a Wide Range of Frequencies".
A key technique that allows a microwave network analyzer to measure complex ratios is the technique of frequency translation by sampling. A block diagram of a network analyzer shown in FIG. 9 is helpful in understanding this technique. Sampling as used in a system of this type is a special case of heterodyning, which translates the input signals to a lower, fixed IF frequency where normal circuitry can be used to measure amplitude and phase relationships. The principle is to exchange the local oscillator of a conventional heterodyne system with a pulse generator which generates a train of very narrow pulses. If each pulse within the train is narrow compared to a period of the applied RF signal, the sampler becomes a harmonic mixer with equal efficiency for each harmonic. Thus, sampling-type mixing has the advantage that a single system can operate over an extremely wide input frequency range.
In order to make the system capable of swept frequency operation, an internal phase-lock loop keeps one channel of the two-channel network analyzer tuned to the incoming signal. Tuning of the phase-lock loop is automatic. When the loop is unlocked, a search oscillator tunes the voltage tuned oscillator back and forth across a portion of whatever octave-wide frequency band has been selected by the user. When any harmonic of the voltage tuned oscillator frequency falls a predetermined interval, e.g., 20 MHz, below the input frequency, i.e., when f.sub.in - nf.sub.osc = 20 MHz, the loop stops searching and locks.
The IF signals reconstructed from the sampler outputs in both channels are the same frequency, e.g., 20 MHz, but, since frequency conversion is a linear process, these signals have the same relative amplitudes and phases as the microwave reference and test signals. Thus, gain and phase information are preserved, and all signal processing and measurements take place at a constant frequency.
The IF signals are first applied to a pair of matched AGC (automatic gain control) amplifiers. The AGC amplifiers perform two functions: they keep the signal level in the reference channel constant, and they vary the gain in the test channel so that the test signal level does not change when variations common to both channels occur. This action is equivalent to taking a ratio and removes the effect of power variations in the signal source, of frequency response characteristics common to both channels, and of similar common-mode variations.
Typically, before the signals are sent to the display unit, a second frequency conversion, e.g., from 20 MHz to about 278 kHz, is performed. To obtain the desired amplitude ratio and phase measurements, the phase-gain indicator contains a linear phase detector and an analog logarithmic converter. Ratio (in dB) and relative phase can be read on a meter of the display unit or de-coupled voltages proportional to gain (as a linear ratio or in dB) and phase for display on the vertical channels of an oscilloscope or X-Y recorder may be provided. If the horizontal input to the oscilloscope or recorder is a voltage proportional to frequency, the complete amplitude and phase response of the test device can be displayed.
Previous network analyzer systems have made network measurements over octave frequency ranges. Various techniques were employed to achieve a complete display when analyzing multi-octave circuit performance. One technique was to record multiple measurements and then concatenate the resulting displays. Alternatively, the sweep was taken over a two- or three-octave range while sweeping very slowly. Slow sweep rates minimize the unusable portion of the displayed data which occurs as the system continues sweeping when the phase-lock loop is unlocked and acquiring a new lock-point. In these ways, attempts were made to minimize the perturbations created in the display during a switch between harmonic numbers by the network analyzer phase-lock loop. Nevertheless, these perturbations frequently resulted in anomalous indications on the display which obscured or limited the value of the data displayed. Hence, one problem in acquiring truly broadband coverage in a network analyzer system was the need to relock on a new harmonic number for each octave in the frequency range covered.
Another problem was that sweep oscillators (sweepers) having truly broadband sweep ranges frequently employ a sequence of smaller bands. This is sometimes accomplished by using a single oscillator followed by an harmonic multiplier to produce the higher frequencies (see, for example, the Hewlett-Packard Journal, March 1975, "A High Performance 2 to 18 GHz Sweeper"). Therefore, during the switch by the sweeper to the next frequency band the network analyzer system would again become unlocked.